In the manufacture of a semiconductor device or flat panel display, plasma processors are used often to perform processes such as formation of an oxide film, crystal growth of a semiconductor layer, etching, and ashing. Among the plasma processors, a high-frequency plasma processor is available which supplies a high-frequency electromagnetic field into a processing vessel and ionizes or dissociates a gas in the processing vessel, thus generating a plasma. The high-frequency plasma processor can perform a plasma process efficiently since it can generate a low-pressure, high-density plasma.
FIG. 13 is a view showing the overall structure of a conventional high-frequency plasma processor. This plasma processor has a processing vessel 1001 having an upper opening. A table 1003 to mount a substrate 1004 thereon is fixed to the central portion of the bottom surface of the processing vessel 1001. Exhaust ports 1005 for vacuum evacuation are formed in the peripheral portion of the bottom surface of the processing vessel 1001. A gas introducing nozzle 1006 through which a gas is to be introduced into the processing vessel 1001 is arranged in the side wall of the processing vessel 1001. The upper opening of the processing vessel 1001 is closed by a dielectric plate 1007. A flat antenna 1015 is disposed on the dielectric plate 1007. The flat antenna 1015 is connected to a high-frequency oscillator 1011 through a waveguide 1014.
A high-frequency electromagnetic field generated by the high-frequency oscillator 1011 is supplied into the processing vessel 1001 through the waveguide 1014 and flat antenna 1015. In the processing vessel 1001, the supplied high-frequency electromagnetic field ionizes or dissociates the gas introduced from the nozzle 1006 to generate a plasma, thus processing the substrate 1004.
In the waveguide 1014, the load-side impedance seen from the power supply side changes while the gas is ionized or dissociated to generate the plasma. Even when the impedance is matched between the power source and load before the plasma generation, if the impedance of the load changes by the plasma generation, impedance matching cannot be performed, and the high-frequency electromagnetic field cannot be supplied into the processing vessel 1001 efficiently. In view of this, an automatic matching device which automatically matches the impedance between the power supply side and load side has been proposed.
FIG. 14 is a block diagram showing an arrangement of the automatic matching device. The automatic matching device includes a load matching unit 1016 provided to the waveguide 1014, a driver 1017 for the load matching unit 1016, a detector 1018 similarly provided to the waveguide 1014, and a controller 1019 which controls the driver 1017 for the load matching unit 1016 on the basis of an output signal from the detector 1018.
The load matching unit 1016 includes a plurality of stubs projecting from the inner wall surface of the waveguide 1014 in the radial direction. For example, the load matching unit 1016 includes three stubs disposed in the axial direction of the waveguide 1014 at a pitch of about λg/4, and three stubs disposed to oppose the three stubs. Note that λg is the tube wavelength of the high-frequency electromagnetic field propagating in the waveguide 1014. The stubs are metal cylinders. The reactances of the stubs change in accordance with the projection lengths by which the stubs project from the inner wall surface of the waveguide 1014 in the radial direction, and accordingly the reactance in the waveguide 1014 changes. The projection lengths of the stubs can be freely changed by the driver 1017 for the load matching unit 1016.
The detector 1018 includes a plurality of probes projecting from the inner wall surface of the waveguide 1014 in the radial direction. For example, the detector 1018 includes three probes disposed in the axial direction of the waveguide 1014 at a pitch of about λg/8. The detector 1018 detects the power of the high-frequency electromagnetic field in the waveguide 1014 which is extracted by the respective probes, and outputs the detection result to the controller 1019.
The controller 1019 calculates the load-side impedance from the output signal of the detector 1018, to obtain the projection lengths of the stubs that satisfy the conditions under which the impedance matches between the power supply side and load side. On the basis of the obtained result, the controller 1019 controls the driver 1017 for the load matching unit 1016 to adjust the projection lengths of the stubs, thus matching the impedance between the power supply side and load side (for example, see International Publication No. 01/76329 pamphlet).
The conventional plasma processor often uses an inexpensive magnetron as the high-frequency oscillator 1011. The magnetron, however, has the following drawbacks.
According to the first drawback, the oscillation frequency distribution of the magnetron has a breadth. Even when the magnetron is operated under the same conditions, a center frequency fc changes as time passes. For example, as shown in FIG. 15A, when the center frequency fc is 2.45 GHz, sometimes the oscillation frequency distribution has a breadth of about ± several ten MHz.
According to the second drawback, the center frequency fc changes in accordance with the output power. For example, as shown in FIG. 15B, even if the center frequency fc is 2.45 GHz when the output power of the magnetron is 1.5 kW, when the output power is changed to 3 kW, sometimes the center frequency fc changes to 2.46 GHz to 2.47 GHz.
If the oscillation frequency distribution of the high-frequency oscillator 1011 has a breadth, the high-frequency electromagnetic field includes a frequency component which is different from the center frequency fc, and accordingly includes a frequency component λ1 which is different from the tube wavelength λg of the waveguide 1014 corresponding to the center frequency fc. In addition, when the magnetron is operated for a long period of time or the output power is changed to change the center frequency fc, the distribution of the tube wavelength component of the waveguide 1014 changes accordingly. Consequently, the frequency component λ1 which is different from λg increases.
The automatic matching device shown in FIG. 14 is designed with reference to the tube wavelength λg of the waveguide 1014 corresponding to the center frequency fc of the high-frequency oscillator 1011. In the above example, when the probes of the detector 1018 and the stubs of the load matching unit 1016 of the automatic matching device are disposed at the pitches of about λg/8 and λg/4, respectively, appropriate control operation based on an accurate detection result is done to allow impedance matching.
Regarding the frequency component λ1 which is different from λg, the probe pitch of the detector 1018 and the stub pitch of the load matching unit 1016 are not λ1/8 and λ1/4, respectively. For this reason, the detector 1018 outputs a detection result including an error. The controller 1019 obtains impedance matching conditions on the basis of the detection result including the error by setting the stub pitch of the load matching unit 1016 to λ1/4. The projection lengths of the respective stubs are adjusted on the basis of the calculation result. Therefore, accurate impedance matching cannot be performed.
If a magnetron which has a breadth in its oscillation frequency distribution and in which the frequency stability is poor is used as the high-frequency oscillator 1011, sometimes the wavelength component λ1 which is different from the tube wavelength λg becomes dominant in the high-frequency electromagnetic field. Then, even with an automatic matching device, accurate impedance matching cannot be performed, and the high-frequency electromagnetic field cannot be supplied into the processing vessel 1001 efficiently.
In order to solve this problem, an oscillator which has a narrower frequency band than the magnetron and good frequency stability may be used as the high-frequency oscillator 1011. An example of such an oscillator can include a high-output transistor oscillator, klystron, and the like. These oscillators, however, are more expensive than the magnetron. If such an oscillator is used as the high-frequency oscillator 1011, an inexpensive plasma processor cannot be provided.